Means for producing an effective short circuit



Nov. 26, 1940 ALFQRD 2,223,160

MEANS FOR PRODUCING AN EFFECTIVE SHORT CIRCUIT Filed Sept. 27, 1938 2Sheets-Sheet l INVENTOR ANDREW ALFORD ATTORNEY I NOV. 26, 194$). ALFQRD2,223,160

MEANS FOR PRODUCING AN EFFECTIVE SHORT CIRCUIT- Filed Sept. 27, 1938 2Sheets-Sheet 2 VIRTUAL SHORT |==8 l l 1( Q1 X VIRTUAL $HORT INVENTOR FANDREW ALFORD ATTORNEY Patented Nov. 26, 1940 UNITED STATES PATENTOFFICE 7 MEANS FOR PRODUCING AN EFFECTIVE "SHORT CIRCUIT ApplicationSeptember 2'7, 1938, Serial No. 231,923

8 Claims.

My invention relates to high frequency transmission circuits and moreparticularly to a high frequency network which may be used in connectionwith a high frequency transmission line to produce the desirable effectsobtained by the use of short circuiting bars without the attendingdisadvantages common to the use of short ,circuiting bars in thesesystems.

In the prior art structures for high frequency 10 transmission systems,it has often been found desirable to terminate certain line sectionswith short circuiting bars. However, such terminations have certaindisadvantages due to the fact that a short circuiting barwillnotyoperate as a complete short circuit at these high frequenciesand as a result considerable voltage is bound to exist in conductorsextended beyond these short circuiting bars.

In accordance with my invention these difficulties are substantiallyeliminated by substituting a form of network comprising crossconnections between the conductors of a transmission, line in place ofthe ordinary short circuiting bar. By the use of such a network thevoltage in the conductors beyond the network may be reduced to asubstantially negligible value or entirely eliminated. 1

It is an object of my invention to provide network which will producethe advantages of a direct short circuiting bar without the attendingdisadvantages thereof.

It is a further object of my. invention to provide high frequencycircuits utilizing the network in accordance with my invention. I

It is a further object of my'invention to'provide a so-called metallicinsulator circuit utilizing networks in accordance with my invention inplace'of short circuiting bars.

. Other objects, advantages and, uses of my in- 40 vention will beapparent from a reading of the particular description of myinvention-made in connection with the accompanying drawings, in which rFig. 1 is a diagrammatic showing illustrating the problem which isovercome by the networks in accordance with my invention; v

Fig. 2 is an illustration of a network'in accordance with my invention;

' Fig. 3 illustrates a preferred form of my invention designed to veryaccurately produce the substantial effect of a short circuitingarrangement; I 1

Figs. 4 and 5 are top and side views respectively, of the network asshown in Fig.3;

Fig. 6 is a diagram of a transmission line pro- .bar' is in the order often amperes.

these voltages are often higher than 220 volts because the currents insuch short circuiting bars 30 vided with a network in accordance with myinvention, for the purpose of explaining the effect'of the network; I

Fig- '7. illustrates an adaptation of my invention to the constructionof so-called metallic in- 5 sulators;

Fig. 8 illustrates the application of my invention to a conjugatenetwork arrangement;

Fig. 9 illustrates an application of myinvention to the tuning of aparasitic reflector; and 10 Fig. 10 illustrates an application of myinvention for use in connection with discriminating networks utilizingcoupled sections.

In Fig. 1, ll represents a high frequency transmission line across whichis bridged a short 15 section of transmission line l2, terminated at asuitable distance from the transmission'line possible to light a 220volt lamp such asindicated at I5 by connecting it across the conductorsextended beyond the short; the spacingbetween the conductors beingapproximately 12" and the current through the short circuiting Inpractice are frequently more than ten amperesl The practicaldifficulties to which voltages penetrating beyond these bar shorts arelikely to lead, are quite'varied and numerous. For example, in the caseof .a conjugate section ar-- 35 rangement similar to that illustrated inFig. 8 of the drawings, in which short circuiting bars are used, twoprincipal types of difiiculties arise.

These types are, one, theimpedance of the section is not altogetherindependent of the length 40 of the wires which extend beyond the barshorts and consequently the tuning is affected by what may be done tothese wires after the position of the short cir-cuiting barhasbeenproperly adjusted. If the wires beyond the shorts are45 grounded attwo different points, as is often convenient from a practical point ofview to keep the short circuitingbar and extending into'the conductors.j

These various difficulties and inconveniences '55 If theenergytransmitted over which arise with the ordinary shorts are obviated byuseof a network in accordance with my invention. This network is in realityno short circuit-at all in the usual sense of the word, but 5 is asimple network which prevents waves from passing to points beyond it.However, for convenience in referring to the network, I refer to it asan X type short. It can be shown that if a network is inserted betweentwo spaced points in a transmission line and the line was conthat acomplete stopping of high frequency energy along the transmission linemay be accomplished by making the two arms of the networkof such lengththat their difieren ce is equal to 180 electrical degrees. Bearing thisfact in mind, the construction and explanation of the so-called X typeshorts will be given with reference to Fig. 2 of the drawings. I

In this figure 2l2 represents a high frequency transmission line whichmay be a line section such as that shown at l2 in Fig. l. The network inits simplest form may consist of two diagonal wires 23, 24, connectedacross line 212 in such a way as to cross and form an .X. ,These wiresare arranged so as not to make electrical contact at their intersectionand may be kept apart by any suitable means such as an insulatingspreader similar to shown in Fig. 3. The length of the wires 23, 26,should be generally equal to several times the spacing of theconductorsof transmission line 212 and these wires 23, '24, should be ofequal length. v v V v The operation :of this network may be'under- 35stood if it is considered as a network in which the line is made toreenter upon itself with a 189 phase shift. Thus the section oftransmission line 2I2 between the points of connection 26, 27,:may beconsidered as one limbA of the network and the conductors 23, 24,- asthe other limb B of the network. The electrical length of the limb Acomprising the line conductors is equal to the distance between 26, 27,in electrical degrees. The electrical length of limb B comprising theconductors 23, 24, is equal to the length of these two conductors'inelectrical degrees 180", because of the transposition at the point ofcrossing. The electrical perimeter with the'spacing of the conductors ofthe transmissionline 2 l 2. The longer the members A and B are made thesmaller is the d'iiference between their length, and consequently thecloser is V to 180.

will arrive at point 27, approximately 180 out of phase with respect tothe current along path B and consequently will substantially neutralizeat this' point. This effect of cancellation may be made absolute bymaking, the length of arms A and B equal. i

' A more accurate arrangement of the X type short or network isillustrated in Fig. 3. In this 7 figure the'vinsulator 35- is made quitelong and thick andthe conductors 23, 22, are drawn tight so that thewhole'network will assume the shape, indicated in Figs. 4 and 5. Such anarrangement makes it possible for the limbs A and B to have 7 morenearlyequal surge impedances and more tinued from the junction of these spacedpoints.

v It can be readily seenthat the current 1 along 2l2 which travelsdirectly along path A nearly equal lengths so that the theoreticalconditions are more closely satisfied.

In practice it has been found that these refinements, such s shown inFigs. 3 to 5, are generally not nece ary and that the arrangement suchas shown in Fig. 2, in which the length between points 26' and 27 isequal to three or four times the spacing between the lines provides suchexcellent cut-ofi that it is usually good enough.

Although in the preferred form the cross connecting wires and theconductors between the points of connections are respectively of equallength'it is evident that the lengths may be unequal it means are usedto compensate for these inequalities.

The network when so constructed may be considered as serving to replaceshort circuiting bars.

At someplace in the network occurs a phenomenon which may be termed avirtual short, sinceit may be considered that if the network wereterminated by a perfect short circuiting bar X. By the term virtualshort is meant that point along the line at which a zero potential maybe assumed in making calculations.

- In order to more clearly explain this point reference is made to Fig.6 in the drawings. In this figure a high frequencysource Si is connectedto a transmissionline T terminated into an X type short which produces acomplete reflection of waves. The standing waves due to this reflectionare indicated by the curve P. The position of thevirtual short may bedefined as the point (located in the X network) which is M4 from thenearest position of the current minimum. The position of this currentminimum may be obtained from the table given. below:

' Table fDistalg tis v nisegw;

I011]. 1 2 IOHI PlQl to first i I 5 t0 first 2' min. max.

Degrees Degrees Degrees Degrees 5 87. 5 2. 5 177. 5 10 85. 0 5. 0 175. 01'5 82. 5 7. 5 172. 5 20 80.0 10. 0 170.0 30 74. 0 16. 0 164; 0 40 67. 023. 0 157. 0

This table represents calculations made in connection with a similarform of network, assumthat the difierence V equals 180.

From thistable it may be seen that for X type shorts of usual dimensionsthe virtual short is located just beyond the crossing point of contheseconclusions agree. very closely with the 7 table so that'this table maybe used as a guide for constructing such networks in most cases. Whenextreme precision is desired the exact position of the virtualshort maybe established by 23, 24 at the point of virtual short is by no meanszero. Likewise the voltage between the conductors of thetransmissionline section between the connection points of conductors 23,24 is different from zero at this point. Both of these voltages dependupon the length of the conductors'in' terms of wavelengths. 'The pointat which nearly zero voltage occurs across the line for the first timeis the end of the X network, that is, at points QlQ}. This point is notreally a point in'the network at all since the voltage across the lineremains nearly zero at all points beyond the X short. I Furthermore thispoint has no significance such as for example, the virtual short has, sothat it is no more than the far terminal of the networks. I

The network or X type short may be utilized in construction of metallicinsulators, as shown in Fig. '7. In this arrangement the transmissionline is supported and spread by means of the conductor section 12.Section 12 if provided with a complete short circuit at a distance aquarter of a wavelength below the transmission line which will have noeffect upon the transmission of waves along the line. Thus, if thesewires are made of rigid material they may serve as an.

insulating spacer. In accordance with my in vention a network or X typeshort as shown at I3 is provided, such that the virtual short of thenetwork is spaced at a distance equal to a quarter of a wavelength fromthe transmission line. With this arrangement the circuit I2 may serve asa spacing insulator, as well as a supporting insulator for thetransmission line 2I2.

Similarly, the X type short may be utilized to replace the shorting barsin conjugate section arrangement, such as shown in Fig. 8. In such aconjugate arrangement the networks 82 may serve to introduce certaindisturbances in the line, and the network 83 may be so designed as toeliminate these disturbances, thus serving to cause a disturbance in theintermediate section of the line, which may be used to produce asuitable phase shift or other desirable changes in the waves transmittedalong the transmission line 2I2. By using the X type short in place ofordinary short circuiting bars, changes in the system due to any effectupon the conductors of networks 82, 83 at the points beyond the shortcircuit and the other difficulties outlined above may be avoided.

In Fig. 9 a still further application of networks in accordance with myinvention is disclosed. In this arrangement 9! represents a transmittingantenna energized from source 92. Spaced from this antenna and insulatedthereform, is the parasitic reflector antenna 93. In order to obtain theproper effect of this parasitic reflector, the transmission lineconnected thereto may be tuned by means of a network in accordance withmy invention instead of an ordinary short circuiting bar. Such a networkis shown at 94. Since the X type short or network substantially preventsany voltage developing in the wires 95 beyond this network, there islittle danger of sary between the conductor 95 and the bar 96. Whereordinary shorting bars are used high voltages may occur necessitatingthe use of other short circuiting bars. The use of an additional shortcircuiting bar will change the characteristics of the system requiring acomplete retuning of the reflector.

A further application of a network or X type short, in accordance withmy invention, is shown in connection with Fig. 10. It has been foundthat when it is desirable to couple a high frequency source such as I orIOI, with an antenna such as I92 or any other suitable load connected toa transmission line such as I93, that a substantial amount of couplingbetween the line and auxiliary networks I04, I95 may be obtained byproviding a shortcircuit or other suitable connection across theconductors I03 at a suitable point. This point should be so chosen thatwith respect to the particular wavelengths used at I00 and NH, a highdegree of coupling is had between the networks I04, I and the.transmission line I 83 at their respective frequencies, butsubstantially no coupling is obtained at other frequencies. In such anarrangement if an ordinary short circuiting bar is used the samedifiiculties arise as was manifested with respect to the supportingarrangement shown in Fig. 9. By utilizing an X type network I96 a'sshown in Fig. 1-0, these difiiculties are. avoided and changes to theconductors between the network I96 and the supporting arrangement IIl'I,is immaterial. In all these arrangements it should be remembered thatthe network should be so connected that the virtual short occurs at thepoint ordinarily occupied by the shorting bars.

While I have disclosed the preferred form of network in accordance withmy invention, together with a number of practical applications thereof,these illustrations are intended to constitute merely a disclosure of myinvention and not as limitations on the scope thereof. What I considermy invention and upon which I desire to obtain protection is defined inthe appended claims.

I claim:

1. A high frequency system comprising a two wire high frequencytransmission line and means for producing a virtual short circuitbetween said conductors with respect to a particular frequencycomprising two conductors of substantially equal length connectedbetween said wires and crossing each other the lengths of saidconductors and their positions relative to one another along said wiresbeing such that the portions of said transmission line between points ofconnection is substantially equal to the length of either of saidconductors to produce a virtual short circuit for said particularfrequency across said transmission line at a point other than those atwhich said conductors are connected.

2. A high frequency network comprising a pair of spaced conductors, anda pair of diagonally arranged conductors of substantially equal lengthcrossing substantially at their centers and insulated from each otherconnected between said spaced conductors the points of connection ofsaid diagonallyarranged conductors being separated by a distance equaltoat least three times the spacing between said spaced conductors- 3. Ahigh frequency network for use in a two conductor transmission line,comprising a pair of substantially straight crossed conductors in- Isulated from one another and connected between the conductors of saidtransmissiOn line atspaced points, and the section of transmission'linebetween said spaced points, separated by a distance equal to at leastthree times the separation of the two conductors of said transmissionline.

4. A high frequency system comprising a transmission line connected tomeans for' producing high frequency standing waves on said line, and anetwork for producing a zero potential in the line at points beyond saidnetwork, comprising a pair of substantially straight, equal lengthcrossed conductors connected across said transmission line at a pointdetermined by said standing wave minimum, said network including aportion of said transmission line substantially equal in length to saidconductors to produce a virtual short at a point a quarter wavelengthfrom said minimum;

5. A high frequency network comprising a first pair of conductors ofequal length arranged in lateral spaced relation with one another, and asecond pair of conductors of equal length and of substantially the samelength as said first pair of conductors insulated from one another,arranged incrossed relationship with respect to one another andconnected between the ends of said .first conductor pair, the saidlengths being at least three times the said lateral spacing.

' 6. A high frequency network comprising a first pair of conductors ofequal length arranged in lateral spaced relation with one another, asecond pair of conductors of equal length and of substantially the samelength as said first pair of conductors arranged incrossed relationshipwith respect to one another and connected between the ends of said firstconductor pair, and insulating means serving to maintain separation ofsaid first pair of conductors and to insulate said second pair ofconductors from each other at their point of crossing, said conductorlength being at least three'times said lateral spacing.

7. A high frequency systemcomprising a two conductor transmission linefor waves of a particular high frequency, and means for producinga'virtual short circuit with respect to said frequency comprising a pairof crossed conductors of substantially equal length connected betweensaid transmission line conductors at points spaced apart a distance atleastthreetimes the separation of said line conductors to establish avirtual short circuit at a distance a quarter wavelength of saidfrequency from a standing: wave minimum in said line. I

8. An isolating support for a two wire high frequency transmission line,comprising a pair of spaced conductors connected respectively to theseparate conductors of said line, a pair of substantially/equal lengthcrossed conductors connected at their ends to the conductors of saidpair,=said crossed conductors being long relative the spacing of saidconductors, said crossed conductors crossing each other at a pointslightly lessthan a quarter wavelength of said high frequency from saidhigh frequency line.

ANDREW ALFORD.

